1. Field of the Invention
The present invention relates to hot roll fuser operations and procedures in a toner fixing station associated with xerography, electrophotography or the like. More particularly, the present invention relates to xerographic types of processing wherein a hot roll fuser and a backup roller are moved into and out of circumferential engagement at various stages associated with the fixing of toner particles on a copy sheet passing between such rollers. The present invention is particularly useful in xerographic copying apparatus using dry release hot roll contact fusers with backup rollers which are utilized to aid in the control of the hot roll temperature.
2. Description of the Prior Art
In xerographic processing, an image is transferred to a copy medium such as copy paper by means of a toner which is usually a pigmented thermoplastic resin. These toner particles are not firmly attached to the copy medium until they have been softened under heat. This softening requires heating of the toner to a relatively high temperature, usually in excess of 200.degree. F. One method of so heating the toner is by hot roll contact fusing. The hot roll fuser operates to pass the toned copy sheet through a fusing nip formed by a heated and driven fuser roller and a movable backup roll. As a result of this contact fusing, the softened toner causes the toned side of the copy paper to tend to ahere to the surface of the hot roll. Thus the copy paper sheet tends to follow the hot roll instead of continuing on the intended paper path subsequent to the fuser station.
A prior art solution to this paper sticking problem is to employ a thick deformable elastomer coating on the hot roll while the backup roll is provided with a rigid surface. Consequently the closing of the fusing nip results in a footprint deformation into the thick, soft coating of the hot fuser roll. An advantage of such a structure is that the nip configuration of the deformable hot roll provides a contour shaped so as to aid in release of the toned side of the paper from the hot roll. The shape of this nip is such that the paper is literally pushed away from the hot roll upon exit from the nip thereby tending to overcome the tendency of the hot toned sheet to stick to the surface of the hot roll.
With such fuser configurations, it is necessary that the hot roll metal core be maintained at a temperature higher than the optimum fusing temperature. This is true since a copy run involving a number of sheets requires that sufficient heat be supplied through this relatively thick heat insulator coating to prevent the fusing nip temperature from dropping to a temperature below the optimum fusing level. However, the use of the thick elastomer coating on the hot roll allows the hot roll fusing surface to achieve unduly high temperatures, substantially above the optimum fusing temperature, when in a standby condition. That is, during idle periods when fusing is not being performed, the thick elastomer coating which is a good heat insulator will approach the temperature of the hot roll metallic core although the two temperatures never reach parity as a practical matter.
In defensive publication T947,012, entitled "Dry Hot Roll Fuser Having Early Fusing Nip Closure" by F. Y. Brandon and J. F. Zimmer, published June 1, 1976 (947 OG 15), apparatus is shown for successfully overcoming the problems in a dry release hot roll fuser where the hot roll is coated with a thick deformable elastomer or the like by providing early closure between the hot roll fuser and the backup roll. Thus, even though the external surfaace of the elastomer coating reaches an unduly high temperature during a standby period, the early closure of the fusing nip causes this external surface to be cooled much as it would be cooled by the fusing of copy sheets. By the time the first sheet to be fused arrives at the fusing nip, the temperature at the fusing nip has lowered to the vicinity of the optimum fusing temperature and adequate fusing occurs without release failure. This is effective since the rigid backup roll is constructed and arranged so as to have characteristics which more or less simulate the cooling effect of a sheet to be fused.
Although the hot roll adherence problem is resolved by the aforementioned prior art apparatus, paper feed failures still may occur particularly if the backup roll temperature reaches an elevated level such as in excess of 200.degree. F. For reasons mentioned in T947,012, early roll closure on the first copy of a set is a desirable fuser property. However, whenever certain usage of the copier is encountered, such as sequential runs of one copy each, the hot roll and the backup roll will be in closure for extended periods causing the backup roll to overheat. This results eventually in the copy sheet tending to follow the backup roll instead of proceeding on its intended paper path thereby creating a jam. The backup roll sticking problem is especially aggravated when the second side of a duplexed copy sheet is being fused.